Radiation 101: How far will radioactive water leaking from nuclear plant go?

Radiation is contaminating seawater near the Fukushima Daiichi nuclear complex, but workers are reported to be making headway sealing the leak. Officials say radioactive substances will dissipate in the Pacific.

Tokyo Electric Power Co.'s Fukushima Daiichi Nuclear Power Plant's No.1, No.2, No.3 and No.4 (r. to l.) reactors are seen in Fukushima Prefecture, northeastern Japan, in this March 31 file photograph released by Japan's Defense Ministry on April 1.

Seawater near the stricken Fukushima Daiichi nuclear complex is highly contaminated with radioactive iodine, plant operator Tokyo Electric Power Co. (TEPCO) reported Tuesday. But TEPCO also said workers are making headway in an attempt to seal a concrete pit they believe is leaking radioactive water into the Pacific Ocean.

Ocean contamination has become a more critical issue in Japan in recent days as the extent of Fukushima’s leakage has become clearer. The presence in seawater samples of highly radioactive substances such as iodine-131 and cesium-137 indicates that the radioactivity is flowing out of reactor units themselves, according to Japanese officials.

This situation led Japan on Tuesday to set first-ever radiation safety limits for fish. That level is equal to the maximum allowable radiation limit for vegetables, said Chief Cabinet Secretary Yukio Edano at a press conference.

“We will conduct strict monitoring and move forward after we understand the complete situation,” said Mr. Edano.

However, TEPCO insists that the radioactivity detected so far presents little risk to human health. The half-life of iodine-131 is eight days, so it will decay quickly. The half-life of cesium-137 is much longer, at 30 years, but it will be quickly diluted in the vast Pacific Ocean, say TEPCO officials.

Where will the radioactive water go? Japan is fortunate in that ocean currents near Fukushima may well carry the radiation away from land and help the dilution process. The Kuroshio Current, the Japanese equivalent of the Gulf Stream, flows up Japan’s east coast before veering off to the northeast in open waters.